Make sure the area to be tattooed is free of any wounds or adverse skin conditions: sunburn, razorburn (when in doubt, don't shave the area- we do that for you anyway).
Use a moisturizer on the area to be tattooed once a day for a week or two prior to getting tattooed to keep the skin supple and hydrated. This improves elasticity and keeps your skin from drying out, especially in the winter.
Remember to drink plenty of water in the days leading up to your tattoo appointment. You should be doing this on a regular basis anyway.
Eat a healthy meal an hour or two before getting tattooed. Tattooing can take a lot out of you and requires a lot of energy.
You can bring a snack with you to munch on during breaks as well as something to drink to keep you hydrated.
Get a good nights sleep the night before.
Try not to stress yourself. A lot of people have great results with meditation or yoga before their appointments. It's worth a try if that's something you're into. Being comfortable and relaxed is the most beneficial.